Controlled Synthesis, Separation And Purification Of Noble Metal Alloy Nanostructures

The ever increasing environmental issues and depletion of fossil resources have stimulated considerable effort to exploit low-cost, clean and efficient renewable power source alternatives. Fuel cells (FCs), which enable the conversion of chemical energy directly into electrical energy by oxidizing renewable fuels, have been considered as ideal alternative power sources for future mobile and stationary applications due to their high energy densities and power densities.However, the high usage and cost of noble metal catalyst and the sluggish kinetics of their electro-catalytic performances are two key obstacles for the commercial viability of fuel cells. Besides, inefficient post treatment methods result in great loss of active materials, which would also increase the producing and using cost of noble metal catalysts.In order to solve the above issues, this thesis focused on controlled synthesis of structural alloy catalysts and invention of advanced post treatment method for such catalysts.Using solvothermal co-reduction method, dendritic Pt3Cu pyramid caps with 3D open porous structures were synthesized. Such complex 3D networks show single crystalline structure with {111} facets exposed. By tuning the growth kinetics, we could also synthesize single-crystalline dendritic Pt3Cu nanocubes with {100} facets exposed.Tuning the reduction kinetics of metal precursors by halogen ions was the key for the formation of alloy structure:? ions could coordinate with Pt? to form a more stable PtI42- and at the same time reducing Cu? to Cu? species, which is easy to be further reduced to Cu0. Therefore the reduction of Cu? and Pt? occurred almost simultaneously and thus alloyed structures formed. The formed alloyed nuclei would further induce zigzag growth pattern of dendritic skeleton while galvanic reaction Pt2+ and Cu0 resulted in formation of new branches at the convex sites of zigzag primary branches, as evidenced by EDS elemental analysis.Based on coreduction-galvanic reaction mechanism, such single crystalline dendritic structure could be extended to multimetallic alloy system and it is possible to tune the compositions in a wide range and meanwhile keep the dendritic cubic morphology.The 3D open porous but interconnected dendritic structure provide bimetallic and multimetallic alloys with ultrahigh electrochemically active surface area (ECSA) while spcific single crystal and facets exposure made such alloy nanostructure ultrahigh electrocatalytic performance. As calculated, the synthrsized Pt3Cu dendritic pyramid caps show more than 3 times and 7 times higher methanol oxidation (MOR) and formic acid oxidation (FAOR) activity than commercial Pt/C, while Pt3Cu and PtCuNi nanocubes show respectively 5 times and near 10 times higher methanol oxidation (MOR) and oxygen reduction (ORR) activities, demonstrating the importance of spatial and crystalline structure design for the enhancement of electrocatalytic performance. More importantly, uncover of the formation mechanism of such complex dendritic single-crystalline nanocry stalls offers a promising synthetic strategy for geometry and crystalline control of multimetallic nanocrystals with tailored physical and chemical properties.Besides the enhancement of intrisic electrocatalytic activity, post treatment of catalysts would also influence the practical electrocatalytic performance. However, traditional post treatment methods have low efficiency, high energy cost and would cause loss of active matarials by repeated wash procedure. In order to conquer these problems, density gradient centrifugation within water/oil interfaces was proposed to purify colloidal nanocatalysts. This one-step purification method could remove more than 99.99% impurities while purify more than 99.9% nanoparticles. Meanwhile, such method could concentrate nanoparticle solution to any concentration without aggregation, even approaching concentration limit that nanopartices are closely packed. Besides, such method has selectivity for nanoparticles with different sizes or shapes, which could be used for removal of byproducts of one batch synthesized nanocatalysts. Furthermore, the droplet sedimentation mechanism and Logistic model for particle sedimentation through water/oil interfaces could help setting optimized purification parameters for nearly all kind of nanomaterials.